777G Off-Highway Truck Hydraulic and Braking System Caterpillar

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Personal injury or death can result from machine movement. Place blocks in front of and behind the wheels to make sure the machine does not move while the parking brakes are disengaged.

An internal oil leak in the brake release system will cause the outlet pressure of the parking brake release pump to decrease. An internal oil leak can also cause too much friction between the brake discs and the plates. Too much friction can damage to the brake discs and the plates.

1. Check the brake release system for an internal leak.
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   Illustration 1	g03498740
   Left-hand side view of the frame (A) Hose assembly (B) Hose assembly (C) Tee (D) Adapter assembly

2. Disconnect hose assembly (A) and hose assembly (B) from parking and secondary brake control valve (E). Parking and secondary control valve (E ) is located across from the hydraulic tank on the left side of the frame.

3. Remove and retain tee (C) from adapter assembly (D).

4. Install a gauge that has a capacity of 6890 kPa (1000 psi) pressure and either a gate valve or a needle valve onto adapter (B).

5. Install tee (C) onto gate or needle valve.

6. Install hose assembly (A) and hose assembly (B) onto tee (C).

7. Start the engine. Operate the engine at high idle.

8. Move the transmission control lever to the NEUTRAL position disengaging the parking brake. Check the oil pressure that is shown on the gauge. The oil pressure should be 5033 ± 689 kPa (730 ± 100 psi).

   Note: If there is any oil leakage in the brake system, the gauge may not show a pressure of 5033 ± 689 kPa (730 ± 100 psi) .

9. Close the valve and look at the gauge. If the pressure has not decreased by 690 kPa (100 psi) after 1 minute, the brake release system will not be affected by the oil leakage. Perform the procedure that is in the Testing and Adjusting, "Parking Brake Release Pressure - Test" section of this service manual.

   Note: If the oil pressure in the brake release system decreases by 690 kPa (100 psi) or more after 1 minute, there is too much oil leakage and each component must be checked.

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   Illustration 2	g03499217
   View of the front left wheel (F) Hose assembly (G) Adapter assembly

10. Disconnect hose assembly (F) from the brake housing on the front wheel group.

    Note: Hose assembly (F) is connected to port "P" on the brake housing.

11. Install a gate valve or a needle valve inline to hose assembly (F). Install a gauge that has a capacity of 6890 kPa (1000 psi) to the valve and the brake housing.

12. Start the engine. Operate the engine at high idle.

13. Move the transmission control lever to the NEUTRAL position.

14. Close the valve that is connected to hose assembly (F). Look at the gauge that is connected to the valve and the brake housing. If the oil pressure that is shown on the gauge decreases by 690 kPa (100 psi) in 1 minute, there is oil leakage in the brake assembly. If there is no drop in the oil pressure, there is no damage to the circuit for the left front brake.

15. Repeat Step 10 through Step 14 for the right front brake.

16. Remove the needle valves or the gate valves and remove the gauges. Reconnect hose assembly (F).
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    Illustration 3	g03499221
    View of the rear left wheel (H) Tube assembly (J) Adapter assembly

17. Disconnect tube assembly (H) from the brake housing on the rear wheel group.

    Note: Tube assembly (H) is connected to port "P" on the brake housing.

18. Install a gate valve or a needle valve inline to tube assembly (H). Install a gauge that has a capacity of 6890 kPa (1000 psi) to the valve and the brake housing.

19. Start the engine. Operate the engine at high idle.

20. Move the transmission control lever to the NEUTRAL position.

21. Close the valve that is connected to tube assembly (H). Look at the gauge that is connected to the valve and the brake housing. If the oil pressure that is shown on the gauge decreases by 690 kPa (100 psi) in 1 minute, there is oil leakage in the brake assembly. If there is no drop in the oil pressure, there is no damage to the circuit for the left rear brake.

22. Remove the needle valves or the gate valves and remove the gauges. Reconnect tube assembly (H).

23. Repeat Step 17 through Step 22 for the right rear brake.

24. Remove any air from the hydraulic oil brake system.

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Personal injury or death can result from air in the oil for the brake hydraulic system. Air in the system can prevent complete brake application and it is possible that the wheels on the machine can not be stopped. Air must not be in the oil for the brake hydraulic system to function properly.